1. Field of Invention
The present invention relates to a method for modifying the surface of finely divided silica.
2. Background Information
Problems are encountered in the dispersion of those finely divided silicas known as reinforcing fillers for silicone rubber compositions, for example, fumed silica and precipitated silica, into the organopolysiloxane on which such silicone rubber compositions are based. In particular, the silicone rubber composition may suffer from changes in structure or viscosity during storage when an untreated finely divided silica is mixed into the organopolysiloxane, causing a phenomenon known as "crepe hardening."
Methods known in the art for eliminating these problems include treatment of the surface of the finely divided silica with an organosilicon compound, for example, with hexaethyldisilazane (refer to U.S. Pat. No. 3,635,743) or with hexamethylcyclotrisiloxane or octamethylcyclotetrasiloxane (refer to U.S. Pat. No. 2,938,009).
A method of treating reinforcing silica in an organic solvent with an organosilicon compound containing hydroxyl or alkoxy radicals and an amino compound is taught in U.S. Pat. No. 3,024,126, issued Mar. 6, 1962. A specially treated filler is taught in U.S. Pat. No. 3,847,848, issued Nov. 12, 1974 as being a silica filler containing moieties selected from hydroxyl groups, water, and mixtures thereof with hydroxyl amine, cyclic siloxane, and silyl nitrogen compound.
Another method is known in which hydrophobic silica filler is synthesized directly from alkyl silicate and a hydrophobicizing organosilicon compound (refer to Japanese Kohyo Koho Publn. 57-500438).
However, those methods in which the surface of finely divided silica is treated with the above organosilicon compounds suffer from the problem of low thixotropy, or even the absence of any appearance of thixotropy, on the part of the mixture with organopolysiloxane when a high degree of treatment and an organopolysiloxane composition with long-term storage stability without "crepe hardening" are sought.
At the same time, at low degrees of treatment of the surface of the finely divided silica, while the mixture with organopolysiloxane will be thixotropic, the long-term storage stability will be unsatisfactory.
As a result, no method for the treatment of finely divided silica has heretofore been available which would provide a high degree of surface treatment of the finely divided silica and which would impart both thixotropy and long-term storage stability when the silica is compounded as an organopolysiloxane composition.
The method for synthesizing hydrophobic silica filler disclosed in Japanese Kohyo Koho Publn. No. 57-500438 consists of a synthesis from alkyl silicate, but is not a method for treating finely divided silica. In addition, said method requires large quantities of alcohol in order to react the silicate, water and hydrophobicizing organosilicon compound in the liquid phase, and this alcohol must be removed in a subsequent process. As a consequence, said method suffers from the problem of complexity.